It's about time to turn around once McCormick Place comes into view, otherwise you'll be putting in some major mileage. Turning back at mile marker 10.5 will give you more than six miles total.
If you go farther south on the path, you'll encounter more nature and fewer people. It can be very peaceful up past 31st Street Beach leading into the Hyde Park area, former home of President Barack Obama. There are also flat, fast bike paths to follow all the way off the path, into the South Shore neighborhood and beyond, but you can save that for another day.
Heading back North, the city will really be coming alive and hopefully by now, you're in a comfortable groove. You'll pass lots of smiling faces on the roughly three miles back to your starting point.